Liquid hydrocarbon rocket propellant



J 22, 1957 D. R. CARMODY ET AL 2,778,188

LIQUID HYDROCARBON ROCKET PROPELLANT Filed Dec. 17, 1951 OX/D/ZER INVENTOR. Dan R. Carmody BY Alex Zlefz WQM A TTORNE Y 2,778,388 Patented Jan. 22, 1957 United States Patent hire 2,778,188 LIQUID HYDROCARBON ROCKET PROPELLANT This invention relatesto reaction propulsion. More particularly, it relates to novel fuels that are spontaneously combustible, when contacted with an'oxidizer, for the generation of hot gasesin a rocket motor. W i LRocket propulsion-is now being used to-assist airplanes in'take-oif-or to attain bursts inexcess of that attainable with the regular power plant. Also rocket propulsion is being used .in the military projectile field, whereinan explosive container is air-borne by means of an attached rocket motor; these projectiles may be launched from the earths surface or from an airplane in flight.

Rocket fuels, now in use, are either a single self-contained fuel-monopropellant-which may be either a with nitrogen tetroxide,N2O4.

on auxiliary ignition devices for initiating combustion at very low temperatures.

Very briefly the hypergolic fuel of this invention consists of a liquid hydrocarbon oil which boils from about 235 to 400 F. and which is derived from the pyrolysis of hydrocarbons at very high temperature, very short contact time and relatively low pressure.

The nitric acid oxidizers of this invention are: White fuming nitric acid-abbreviated WFNA-which normally contains less than about 2 weight percent of water. More dilute solutions have been utilized by fortifying the acid Red fuming nitric acid RFNA-normally contains less than about 5 of water and between about 5 and of N204. Sodium and potassium nitrites and sodium and potassium nitrates are often added to WFNA to depress the freezing point;.usu-

I allyan aqueous solution of the salt is used. Liquid nitrogen tetroxide is an excellent oxidizer when used above its freezing point. An excellent oxidizer for use at temperaatures as low as about 65 F. is obtainedby adding 10 to of sulfuric acid monohydratetHzSOr) or about 1 to 30% of oleum to strong nitric acid., The parnitric acid, and nitric acid-oleum mixtures.

solid or a liquid; or a separate fuel and a separate oxi-- dizer-bipropellant. in separate tanks outside the rocket motor itself. The solid monopropellant fuels are stored in theconibustion chamber of .the rocket motor. The bipropellantrocket motor consists of a suitable combustion chamber provided with one or more pairs of nozzles adapted to inject therein the fuel andthe oxidizer, separately and simultaneously. The combustion of the fuel and the decomposition'of the oxidizer creates a mass of hob-burning gases which are ejected at high velocity througha suitable orifice; the reaction from this ejection provides the propulsive force. In general, the bipropellant rocket motor is more amenable to control and uses a somewhat more economical combustion chamber. 1 a L The ignition reaction between the fuel and the oxidizer maybe initiated by an electric spark, a hot wire, ahot surface or may be spontaneous. A spontaneous combustion or self-ignition is preferredbecause of the possibilities of electrical and mechanical failure of thespark and hot surface methods of ignition. Afuel which is self-igniting when contacted with'an oxidizer is called a hypergolic material. j

Many materials which are hypergolic at temperatures of about +75 F lose this property when the temperature is lowered. The temperature'at the earths surface may vary from a high of about +120 F. to a low of -40? F., and in thePolar and sub-Polar regions, was much as 70 F; The temperature of liquids stored in, ordinary jtanks exposedto the sun may reach as much as +l F. The temperatures encounteredby airplanes at high altitude are often as low as F. and, may be lowerthan F. Thus a rocket motor using a hypergolic fuel may have to be started into operation with the fuel and oxidizer. at a temperature as low as, or possibly lower than, 0 F. In this specification, the term atmospheric temperaturesincludes the entirerange from about F. to about '70 F.

An object of this invention is to obtain reaction propulsion by means of a hypergolic fuel and a nitric acid oxidizer. Another object is to provide a fuel for reaction propulsion which is'hypergolieat atmospheric temperatures. Yet another objectis to provide a relatively cheap method of reaction propulsion. A particular object is areaction propulsion method that is not dependent The bipropellant fuels are stored" ticularly effective nitric acid oxidizers contain not more than about 5 weight percent of non-acidic material, such as, water or aqueous. potassium nitrate solution. The pre ferred oxidizers are white fuming nitric acid, red fuming The use of the general term ,nitric acid .oxidizer in this specification and in th'eclaims is intended to include all the favorable compositions described in this paragraph.

One source of the hypergolic fuel of this invention is the product from the pyrolysis of hydrocarbons in the vapor'phase at temperatures from at least about.1250 to 1800 F., at pressures below about 100 p. s. i. a. usually below about 50 p. s. i. a., and at contact times from about 0.05 to 5 seconds, usually below about 2 seconds. Suitable feeds are ethane, propane, butane, propylene, butylene, naphthas, gas oils and other hydrocarbons which can be vaporized at the temperature of pyrolysis without an excessive amount of cokeformation, The high temperature vapor phase pyrolytic reaction is normally used for the production of olefinic gases, such as, ethylene and propylene; and for the production of aromatic hydrocarbons, such as,benzene, toluene and xylene. In general, at agiven temperature, the longer the contact time the greater the amount of aromatics produced. The gases from the cracking reaction are rapidly cooled, usually by quenching with water, to a temperature of about 400 F. A viscous tarry material condenses out of the gases during the quenching. The gases from the quenching opera tion are compressed and cooled; a liquid material which boils between about 100 and 400 F. condenses out of the gases during this compression-cooling step. This liquid is commonly known as dripolene. The amount of tar and dripolene produced is dependent upon the feed, temperature, contact time'and the pressur The preferred operating conditions for the production of the hypergolic fuel of this invention area temperature from about 1400 to l550 F., a contact time from about 0.1 to 1 second, a pressure below about 35 p. s. i. a., and a feed consisting of ethane or propane, or mixtures thereof.

The .total liquid material product is an extremely complicated mixture of hydrocarbons. The tar, which is defined as the material boiling above aboutj400 F., consists mainly, of'naphthalene and alkylated naphthalene.

An appreciable amount of anthracene is present and the remainder is thought to consist of condensed-benzene ring compounds. .This tar is n-on-hypergolic with nitric acid oxidizers at atmospheric temperatures. The presence of tarry material in the dripolene should be avoided because the tarry material is detrimental to hypergolic activity present; about one-half of the dripole'ne consists of benzene, toluene, xylene and ethylbenzene; styrene is present in appreciable amounts.

The material boiling above 300 F. is known to contain some dicyclo'pent'adiene; the remainder is thought to consi's't 'of higher boilin alkylated benzenes, condensed cyclool'efins and cyclodienes; in addition aromatics which have been alkylated with cyclool'efins and/or cyclodienesare thought to be present. When the ASTM end point of the dri'polene is about 400 F. a minor amount of naphthalene is usually also present. The presence of naphthalene is detrimental "to the freezing point of the dripolene and normally the 'dripole'ne is cut to an end point of about 375 F. to'eliminate naphthalene.

It has been discovered that the liquid hydrocarbon fraction of dripol'ene which boils from at least about 250 F. to 400 F. at atmospheric pressures is an extremely effective hypergolic fuel with nitric acid oxidizers. The preferred hypergolic fuel of this invention boils within from about 270 to about 375 F.

While 'the high temperature pyrolysis of hydrocarbons, such as, ethane, propane, gas oil, etc. is the preferred source of the hypergolic fuel of this invention, other sources are available. These other sources produce in abundant supply a light oil fraction which contains a hypergolic fuel equivalent to that obtained from dripolene. The more common sources are'related to the carbonization of coal. A very good source is the light oil obtained.

from the'carbonization of coal at low temperatures, 'i. e., from about 1250" to 1600?). Appreciable amounts 'of hy'pergolic fuel are obtainable from the light oil derived from 'the-so-called high temperature carbonization'of coal for the production of metallurgical coke. An excellent source is the light oil derived from the production 'of coal gas, particularly when this :process is carried 'out at from about 1250 to 1650 F.

Still another source of hypergolic fuel is the drip oil fraction amounted to 3 weight percent of the feed. The non-condensible product contained about 25 volume percent of ethylene and about 11 volume percent of propylene.

The dripolene was characterized as follows:

ASTM Distillation, F.: a 1

Initial Y 106 API e..-e 35 Refractive index, n 1.4830 Bromine number, cg. Bra/g 89.4

Maleic anhydride value, mg. M. A./g 104.1

obtained from the manufacture of producer gas when using coal. An excellentsource of hyperg'olic fuel is the drip oil obtained in the manufacture of carbureted water gas.

It is to be understood that the above list of sources of the fuel of this invention is not complete-and thatthere are other lesser known sources. It is intended that the descriptive phrase high temperature pyrolysis of hydrocarbons includes all the processes-operatingat atemperature of at least about 1250 'F. to produce a liquid hydrocarbon'oil from which can be distilled a fractioncontainin'g polyolefinic linkages and having a maleic anhydride value of at least'abo'ut 35 and'boiling from at least about 250 to'400 F.,'preferablyfrom about 270 to 375 F.

By way of an example, a particular hypergolic fuel suitable for use in rocket propulsion is described below. In *this particular example the feed to the pyrolysis 'consisted of ethane, 8 volume percent; propane, 90%; and butanes, 2%. The pressure at the inlet to the furnace was about 40 p. s. i. g. and the exit pressure Was about 11 p. s. i. g; The transferlinetemperature was 1520" F. and the contact time inthe high temperature zo'nein the furnace was about 0.2 second. The "hot gasesjwer'e quenched with water to eliminate 'ta'r. The dripoleu'e A sample of the dripolene was analyzed by conventional Compound: Volume percent Propane and propylene. .7 Isobutane 0.1 Butylenes 2. n-Butane u 0.4 Butadiene e 4. Pentane 0.4 Pentadiene and eyclopenta-die'ne; 8. Pentene and cyclopent'ene 6. Benzene 35. Toluene r '8. Xylenes '5.

f Styrene 3. Dicyclopentadiene 5.

The ignition characteristics of the dripolene and vari! ous fra'etions thereof werestudied using a drop -test.method.. This method utilizes a teStQtube', 1 in. X 4 in., containing 1 ml. of oxidizer. The fuel is added dropwise into 'the'tes't tube by means of a syringe calibrated in 0.01 ml. markings. Usually 0.1 nil. of fuel is added per test; however, the feed usage may vary-between 0.01 and 0.2 ml. per ml. of oxidizer. Low temperature tests were carried out by cooling the test tube and the oxidizer contained therein to the desired temperature by means of a Dry Ice-chloroform bath; a drying tube inserted into the top of the test tube excluded moisture. The fuel was cooled separately to the desired temperature. By super- Test 1 In this-test"the hypergdlic activity at +75 "F. was determined for individual fractions distilled "from the above described sample of dripolene and the bottoms remaining after eah' overhead 'fraetion had been distilled;

W FNA which contained 2 weight percent of water was the oxidizer. The results ofthis testfare zji Overhead Overhead Bottoms Fraction Volume Ignition Volume Ignition- Numbor Percent Percent 100 None.

29 Very Short Delay.

Overhead fraction 2 is thought to consist mainly of pentadiene and cyclopentadiene. The sharp break between the extremely hypergolic 29% bottoms and' the non-hypergolic 35% bottoms is quite astonishing.

The characteristics of the hypergolic 29% bottoms are,

' Boiling range (1 atm.) F 255-395 Specific gravity 0.975 Refractive index, n 1.5481 Freezing point, F l00 Bromine number 137.4 Maleic anhydride value-.. 38.9

Test 2 The lowest temperature at which the hypergolic 29% bottoms fraction from Test 1 was hypergolic with various oxidizers was determined.

Test 3 For purposes of comparison a sample of commercial grade turpentine was measured as to hypergolic activity in the drop test at +75 F. The turpentine did not ignite with WFNA when as much as 0.2 ml. was used with 1.0 ml. of oxidizer. On the other hand only 0.05 ml. of the 29% bottoms is required for ignition under the same conditions; even at -78 F. (WFNA was supercooled) only 0.12 ml. was required for ignition.

This invention is particularly advantageous when the fuel, oxidizer and combustion chamber are initially at atmospheric temperature as combustion begins without auxiliary ignition devices or without preheating of the combustion chamber. The nitric acid oxidizer and the fuel should be added to the combustion chamber separately and simultaneously so as to contact each other with considerable intermingling action. When using the hypergolic fuels of this invention about 3.5 to 6 pounds of WFNA are injected per pound of the fuel. While it is possible to vary the ratio during operation, it is preferred to maintain a constant ratio.

By way of example, this invention is applied to the driving of an air-to-air missile. The figure shows a schematic layout of the combustion chamber and bi propellant feed system of a reaction motor, such as is used in a military rocket projectile. In the figure, vessel 11 contains a quantity of inert gas under high pressure; nitrogen or helium is a suitable gas. Helium is passed through line 12, through a regulatory valve 13 which passes the helium into line 14 at a constant pressure. From line 14, the helium is passed into line 16 which is connected to the vessels containing the fuel and the oxidizer. Vessel 17 contains the oxidizer; the pressure of the oxidizer results;,i.n the generation of a large volume of very hot gases which pass out of thecombustion chamber through opening 28; the reaction from this expulsion of gases drives the rocket.

The fuel described in Test 1 is used in this example;

RFNA containing about 10 weight percent of N204 is used as the oxidizer because it is liquid at about 70 F. which the oxidizer and fuel will attain while being carried at high altitude by an aircraft looking for a target. The missile is launched by activating the solenoids on valves 21 and 41. The oxidizer and the fuel are forced into the combustion chamber in the weight ratio of 4.0 to l. Instantly combustion takes place and the missile hurtles toward the target.

We claim:

1. A method of generating gas, which method comprises injecting separately and essentially simultaneously into the combustion chamber of the gas generator (1) a hypergolic liquid hydrocarbon fuel boiling over the range of about 250 to 400 R, which fuel is derived from the liquid product of the pyrolysis of a hydrocarbon selected from the class consisting of ethane, propane, butane, propylene, butylene, naphthas, and gas oils in the vapor phase at a temperature between about 1250" and 1800 F. at a cracking zone pressure of not more than p. s. i. a. and for a cracking zone residence time between about 0.05 and 5 seconds by separating a liquid product from other cracking products and distilling said liquid product to obtain said fuel and (2) a nitric acid oxidizercontaining not more than 5 Weight percent of non-acidic materials, in an amount and at a rate sufi'icient to initiate a hypergolic reaction with and to support combustion of the fuel.

2. The method of claim 1 wherein said fuel boils over the range of 270 and 375 F.

3. The method of claim 1 wherein said oxidizer is white fuming nitric acid.

4. The method of claim 1 wherein said oxidizer is red fuming nitric acid.

5. A method of generating gas, which method comprises injecting separately and essentially simultaneously 'into the combustion chamber of the gas generator 1) a hypergolic liquid hydrocarbon fuel which boils over the range of about 250 and 400 R, which fuel has been derived from the liquid product of the pyrolysis of a hydrocarbon consisting essentially of ethane in the vapor phase at a temperature between about 1400" and 1550 F. at a cracking zone pressure of not more than about 35 p. s. i. a. and for a cracking zone time between about 0.1 and 1 second and separating a liquid product from other reaction products and distilling said liquid product to obtain said fuel and (2) a nitric acid oxidizer containing not more than about 5 weight percent of nonacidic materials, in an amount and at a rate sufiicient to initiate a hypergolic reaction with and to support combustion of the fuel.

6. The method of claim 5 wherein said fuel boils over the range of 270 and 375 F.

7. The method of claim 5 wherein said oxidizer is white fuming nitric acid.

8. The method of claim 5 wherein said oxidizer is red fuming nitric acid.

(References on following page) 7 7 References Cited in the file of this patent UNITED STAT'BSI PATENTS OTHER REFERENCES De Rosa: Journal of American Rocket Society, No. 61 (1951), March I945,page 5. r

2,091,375 Pyzel Aiig. 31, 1937 2,200,463 Alexander -M'a'y 14, 1940 2,378,067 Dorsett et a1. June '12, 1945 2,557,018 Viles June 12, 1951 2,563,305 Britten et a1. Aug. '7, 1951 2,573,471 Malina et a1. -1. Oct. 30, 1951 I Military Specification, MIL-F-5624A, May 23,1951, may be obtained from the Commanding GeneraL'Air Materiel Command, Wzight Patterson Air Force Base, Dayton, Ohio; 1 I

Trent et al., Journal of American Rocket Society No. 21 (1951), pp. 129-31. Ab's'ti'acted in Chem. Abstracts 1952 pp. 8373,. 374. g 

1. A METHOD OF GENERATING GAS, WHICH METHOD COMPRISES INJECTING SEPARATELY AND ESSENTIALLY SIMULTANEOUSLY INTO THE COMBUSTION CHAMBER OF THE GAS GENERATOR (1) A HYPERGOLIC LIQUID HYDROCARBON FUEL BOILING OVER THE RANGE OF ABOUT 250* TO 400*F., WHICH FUEL IS DERIVED FROM THE LIQUID PRODUCT OF TTHE PYROLYSIS OF A HYDROCARBON SELECTED FROM THE CLASS CONSISTING OF ETHANE, PROPANE, BUTANE, PROPYLENE, BUTYLENE, NAPTHAS, AND GAS OILS IN THE VAPOR PHASE AT A TEMPERATURE BETWEEN ABOUT 1250* ABD 1800*F. AT A CRACKING ZONE PRESSURE OF NOT MORE THAN 100 P.S.I.A. AND FOR A CRACKING ZONE RESIDENCE TIME BETWEEN ABOUT 0.05 AND 5 SECONDS BYSEPARATING A LIQUID PRODUCT FROM OTHER CRACKING PRODUCTS AND DISTILLING SAID LIQUID PRODUCT TO OBTAIN SAID FUEL AND (2) A NITRIC ACID OXIDIZER CONTAINING NOT MORE THAN 5 WEIGHT PERCENT OF NON-ACIDIC MATERIALS, IN AN AMOUNT AND AT A RATE SUFFICIENT TO INITIATE A HYPERGOLIC REACTION WITH AND TO SUPPORT COMBUSTION OF THE FUEL. 